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The structure and stratigraphy of the Aiguilles Rouges External Massif, its autochthonous Tertiary sediment cover and overlying Morcles nappe in Emosson (Helvetic Realm of the Western Alps, Switzerland)

Michael J. Heap1

1University of Liverpool, 4 Brownlow Street, Liverpool, L69 3GP, U.K.

West face of Petit Dent de Morcles

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Abstract

The stratigraphy of Emosson is dominated by three main groups of rocks: the metamorphic basement (Aiguilles Rouge massif), its autochthonous Triassic cover and a large recumbent fold called the Morcles nappe. The metamorphic basement displays low-grade subgreenschist metamorphism and is folded on both the km and metre-scale. It also contains many high angle normal faults that have provided palaeostress plunge and plunge directions of 093 64 for σ1, 203 10 for σ2 and 300 27 for σ3. Foliations are folded with the km-scale folding, therefore the foliations pre-date the folding. These foliations could relate to bedding in the sedimentary rocks before they experienced Variscan metamorphism. The metamorphic basement in Emosson is very variable and comprises of nine formations in total, but it mainly consists of a granitoid gneiss called augen gneiss. The top of the metamorphic basement is signalled by an augen gneiss formation containing a red/pink weathered top marking an unconformity. Triassic sediments (the Trias) unconformably overlie this augen gniess and consist of sandstone, mudstone and a carbonate conglomerate called cargneule. The sandstone is a very mature quartz arenite and contains many dinosaur footprints on one of its beds; the presence of a mature, well-rounded arenite with faintly dipping laminations indicates the sandstone was formed in a beach environment. Two types of mudstone are present: a green less oxidised mudstone and a more oxidised purple mudstone. The different colours do not reflect different parent compositions but whether the mud was waterlogged or not during deposition. The mudstones lack bioturbation, contain starved sandstone ripples and were formed on the floodplain of a sandy braided river. The cargneule can be split up into two types: one which contains angular polygonal hollows once occupied by halite and one which contains clasts derived from the surrounding material. The formation of the cargneule occurred in an evapouritic sabkha environment. The top of the cargneule is a decollement surface as the Morcles nappe, a succession of limestones, marls and shales, was thrust over it resulting in the burial of the underlying sediments and basement. This thrusting caused slight metamorphism in the Trias, especially in the cargneule where folding related to nappe emplacement is present. The Morcles nappe contains many large recumbent folds, the hinges lines of these folds were formed in close proximity to a shear plane resulting in the ‘shear out’ of the hinge lines to form sheath folds.

Key words: Aiguilles Rouge Massif; Morcles nappe; sheath folds; Triassic sediments; palaeostress.

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Table of contents

1. Introduction………………………………………………………………………5 Location of the mapping area (Emosson)……………………………………5 Alpine history, formation and geology………………………………………7 A brief history of the Alps…………………………………………...7 Western Alp geology and geological setting of Emosson……………8 Aims, scope and outline of the report…………………………………….....13

2. The Variscan metamorphic basement………………………………………….14 Stratigraphy and rock descriptions…………………………………………..14 The small tunnel feldspar gneiss Formation…………………………16 North of the small tunnel layered metasediment Formation…………17 South of the small tunnel mica schist Formation…………………….19 North of the big tunnel garnet gneiss Formation……………...……..21 North of the big tunnel layered metasediment Formation……...……23 Barrage de Barberine augen gneiss Formation…………………...….25 Barrage de Barberine garnet schist Formation…………………...….27 Combe du Col mica schist Formation…………………………….....29 Tete des Ottans augen gneiss Formation………………………….....30 Structural observations within the metamorphic basement……………….....31 Folding……………………………………………………………….31 Faulting………………………………………………………………33 Other observations…………………………………………………..36 Interpretations of the metamorphic basement……………………………….37 Metamorphic grade…………………………………………………..37 Formation of augen gneiss…………………………………………...38 Different formations of augen gneiss………………………………..38 Folding……………………………………………………………….41 Faulting………………………………………………………………44 Implications of garnets in metamorphic formations…………………48 Other interpretations…………………………………………………48

3. The granite……………………………………………………………………….50 The church white granite Formation…………………………………………50 Stratigraphy and rock descriptions…………………………………..50 Interpretations of the granite…………………………………………52

4. The Trias…………………………………………………………………………53 The signpost sandstone Formation…………………………………………..53 Stratigraphy and rock descriptions…………………………………..53 Dinosaur footprints…………………………………………………..58 Col de Barberine interbedded sandstone and mudstone Member……60 Interpretation of the sandstone features………………………………61 The signpost mudstone Formation…………………………………………...63 Stratigraphy and rock descriptions…………………………………...63 Interpretation of the mudstone features………………………………65 The Col du Barberine cargneule Formation………………………………….66 Stratigraphy and rock descriptions…………………………………...66

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The river valley white structureless Member………………………71 The river mouth quartz limestone Member………………………...72 Interpretation of the cargneule features…………………………….73 Palaeoenvironmental analysis for the Trias………………………………..75

5. The Morcles nappe……………………………………………………………..77 The Combe des Fonds carbonate nappe Formation………………………..77 Stratigraphy and rock descriptions…………………………………77 Structural observations within the Morcles nappe…………………79 Interpretations of the Morcles nappe……………………………….82

6. Superficial deposits…………………………………………………………….86

7. Geological history………………………………………………………………87

Acknowledgements and references………………………………………………91

Appendix…………………………………………………………………………...93

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1. Introduction

Location of the mapping area (Emosson)

Emosson is located in the region of Valais, which is located in the southwest of Switzerland (Fig1.2). Emosson was a small village but has been flooded by the consecutive construction of two dams: the Barrage de Barberine in 1925, which was eventually superseded by the Barrage d’Emosson in 1973 (Fig1.1).

Position of the now submerged Barrage de Barberine

Barrage d’Emosson

Lac d’Emosson Figure 1.1 - Photograph of the barrage d'Emosson and Lac d'Emosson (from www.swisscastles.ch)

Figure 1.2 - Map of Switzerland with Valais coloured orange

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Figure 1.3 - Map of Valais showing position of Emosson and Lac d’Emosson; the exact mapping area is the east side of Lac d’Emosson (shaded red)

The area under study is on the east side of Lac d’Emosson (Fig1.3) from Six Jeur (2062m), located near the Restaurant de Gueulaz, as far as Tour Salliere (3219m) keeping on the west side of Bel Oiseau (2628m), Fontanabran (2702m) and the Pointes d’Aboillon.

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Alpine history, formation and geology

A brief history of the Alps

Before the Alps were formed the Hercynian (also known as Variscan or Amorican), a vast mountain belt, covered Europe. During the Permian sediments were deposited unconformably on the eroded basement of this orogen, deposition ensued in the Triassic when mostly carbonate marine rocks were deposited. The Adriatic and Eurasian plates collided following the subduction of the Piemonte Ocean (a branch of the Tethys Ocean that opened due to rifting in the Jurassic), which separated the two continents. This resulted in the subduction of the Eurasian plate under the Adriatic plate and the geology of the Alps thus consists of rocks from both continents and oceanic crust. Subduction of this ocean began in the mid-Cretaceous and was directed down towards the south or southeast; the Adriatic plate therefore overrode the Piemonte Ocean and eventually the Eurasian continental margin (Wheeler, 2003). The Pip or Sliver model is an applicable model for the formation of the Western Alps (Fig. 1.4).

Figure 1.4 – The Pip or Sliver model (a) Piemonte Ocean subducted and French continental crust is 'pulled' down by more dense mantle (b) crust weakens and the movement sense is reversed forcing the subducted continental crust back to the surface

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Western Alp geology and geological setting of Emosson

The main geological units found in Emosson are the Aiguilles Rouge Massif (one of the External Massifs in the Helvetic Realm), its autochthonous Triassic sedimentary cover (mainly carbonates) and a large recumbent fold called the Morcles nappe. More or less allochthonous after the Alpine events, the External Massifs are the relics of a Variscan continental crust that had formed Carboniferous-Permian horsts, within which updomed Variscan nappes are preserved (von Raumer and Schwander, 1984). Overlying the Morcles Nappe is a thin zone of Ultrahelvetic sediments that were thrust over the Morcles’ Triassic to Tertiary sequence during the embryonic stages of the folding responsible for the formation of the Morcles fold. Overlying the Morcles and Ultrahelvetic units are 2-4km of higher Helvetic nappes (Ardon, Diablerets, Mont Gond and Sublage) and more then 5km of now eroded Préalps nappes (Kirschner, Sharp and Masson, 1995). The cross section below shows the positions of the different Helvetic units (Fig 1.5).

Préalps nappes

Ultrahelvetics nappes

Morcles nappe

Aiguilles Rouge massif

Figure 1.5 - Cross section showing the relative positions of the different Helvetic units (modified from Kirschner et al 1995)

Salanfe area two Page 9 (Auberge de Salanfe) Lac du Salanfe

Lac d’Emosson Salanfe area one

Finhaut and French post- Salvan area Hercynian continental, marine shelf, and passive margin cover French Hercynian (Carboniferous- metamorphic Jurassic) basement

Figure 1.6 - Regional geology map of the western Alps (produced by the Bureau de Recherches Geologiques et Minieres) The regional geological setting of Emosson is shown in Fig. 1.6, it shows that some of the formations present in Emosson are fairly continuous over the neighbouring area. To gain a greater understanding of the surrounding area and to see how Emosson fits in with the regional geology, four nearby localities were investigated: two localities in Salanfe, one in the Finhaut and Salvan area and the west side of Lac d’Emosson.

Salanfe area one

Salanfe area one is just south of the Lac du Salanfe (location shown in Fig. 1.6), just from the regional geology map it can be seen that formations can be traced from Emosson to this area so there is much scope for comparison and correlation. The area is essentially very similar to Emosson and contains metamorphic basement (Aiguilles Rouge massif), Triassic sediments and the Morcles nappe. The descriptions of the rock types found within the metamorphic basement are as follows:

Metasedimentary rock – is a predominantly grey, quartz gneiss containing feldspar, muscovite, chlorite and quartz. This metamorphic rock also contains evidence for faulting.

Augen gniess – contains large (5mm) eye-shaped feldspars within a gneissose matrix. At the contact between the metamorphic basement and the Trias the eye-shaped feldspars have weathered red/pink, this signals the unconformity. This rock type is very similar to the augen gneiss seen in Emosson.

Marble – found not in situ within the metamorphic basement; contains dark bands but is mainly white in colour.

Amphibolite – also found not in situ within the metamorphic basement, this rock type is coarse-grained and contains a high proportion of hornblende.

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Pegmatite – is a white and black, coarse-grained igneous rock with a granitic composition.

Some of the formations are similar to those seen in the metamorphic basement in Emosson. However, the presence of marble, amphibolite and pegmatite, rock types not seen in Emosson, demonstrates the high variability of the Aiguilles Rouge massif. Descriptions of rock types found in the Trias are as follows:

Conglomerate – just below the sandstone there is a thin layer (~20cm thick) of matrix- supported conglomerate that contains large angular clasts.

Sandstone – a fairly well sorted, cream-orange sandstone with a high proportion of quartz. The rock also contains cross bedding and ripples.

Cargneule and mudstone were also observed but not investigated in detail due to inaccessibility and time constraints. Apart from the thin conglomerate layer the rock types seen are similar to those in Emosson, this shows that the Trias is a very laterally consistent unit. Since the conglomerate layer beneath the sandstone is not present in situ in Emosson it cannot be included into the generalised vertical section (GVS) for the area. A GVS produced by Demathieu and Weidmann (1982) shows the position of the layer (Fig. 1.7):

Sandstone

Conglomeratic sandstone Weathered basement

Figure 1.7 - Part of the GVS produced by Demathieu and Weidmann (1982) showing the position of the conglomerate layer

Salanfe area two

Salanfe area two is located just north of the Lac du Salanfe (location shown in Fig. 1.6), this area contains mostly Morcles nappe. From this area (Auberge de Salanfe) the other side of Tour Salliere could be seen clearly. It shows numerous recumbent folds and a large S-type fold that can be traced across several of the visible peaks within the Morcles nappe. The cargneule is present in this area directly below the Morcles nappe; it is creamy-yellow in colour and contains 2-4mm angular polygonal hollows. From this area numerous formations within the Morcles nappe

Page 11 could be identified showing much more diversity than seen in Emosson. Descriptions of the formations found in the Morcles nappe are as follows:

Deformed limestone – contains strong foliations that are oblique to bedding, kink bands and pseudomorphed belemnites that have had their soft parts replaced by pyrite and contain pressure shadows of fibrous calcite.

Mylonite – high strain, yellow limestone mylonite that contains foliations and lineations.

Carbonate rock – contains siliclastic, coarse-grained orange sand and strained carbonate that is grey and slaty. It has been metamorphosed slightly, shows reactivation surfaces and contains strained elongate pebbles.

Microfossil layer – is stratigraphically just above the carbonate rock, this layer is well fractured and contains numerous microfossils.

Finhaut and Salvan area

The Finhaut and Salvan area is located to the northeast of Emosson (location shown in Fig. 1.6); this area contains formations that are very different to those seen in Emosson and Salanfe. Descriptions of the observed formations are as follows:

The north of Finhaut granite Formation – Is a grey, coarse-grained massive igneous rock with granitic composition; it also contains large (~10mm) crystals of quartz and similar sized dark grey clasts. It is different in appearance to the granite found in the Six Jeur area in Emosson.

The railway tunnel phyllite Formation – Is a grey, very fine-grained slaty metamorphic rock that contains small flakes of muscovite; it also has slight green colour suggesting the presence of chlorite.

The north of Finhaut breccia Formation – Is a predominantly grey, brecciated rock containing randomly arranged gneissose and schist portions within a more massive matrix. It contains high proportion of shiny flaky muscovite and a few large (~70mm in diameter) rounded clasts of lighter coloured weakly foliated rock (Fig 1.8). This breccia could be present here due to the location of a large fault that may run through the Finhaut/Salvan area.

Figure 1.8 – Photograph of the north of Finhaut breccia Formation Page 12

The track to Salvan massive Formation – Is a creamy-grey, medium-grained, massive igneous rock containing shiny muscovite and feldspar clasts (<50mm).

The west side of Lac d’Emosson

Much reconnaissance work was done on the west side of the lake since it can be simply correlated and it is the most easily accessible area. Grey and weathered orange metasediments, similar to those seen on the east, are present on the west side. However, tracing individual beds such as these across the lake proved problematic. A metamorphic formation on the west side also contains garnets and white needle-like crystals of silimanite, PT and grade indicating minerals such as silimanite are not present on the east side. Fault breccia and augen gniess are also present on the west side. The dinosaur prints of the Vieux-Emosson area are well documented. Five different dinosaur prints were found in total, these were distinguished by observing foot shape, number of toes and depth and style of the footprint. Bronner (1976), who first worked on the footprints, stated that there are nine different species and 800 dinosaur prints in total in the area. Most of the geology can be correlated under the lake and over to the other side, using data from both sides a correlated reconnaissance-scale map was produced to demonstrate how laterally consistent some of the units are (Fig. 1.9):

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Unconformity between augen gneiss and Triassic sediments Tectonic thrust surface between Morcles nappe and Triassic sediments Augen gneiss

The Morcles Triassic nappe sediments

Granite

Figure 1.9 - 1:25000 reconnaissance-scale map showing how the geology from both areas can be correlated (western reconnaissance data from A.Lennie)

Aims, scope and outline of the report

The 35 days spent mapping were used to understand the complex structure and stratigraphy of the area, interpret palaeoenvironment and processes and to form a history of deformation events. Another aim was to understand how the area geologically fits into the bigger picture, concentrating on two neighbouring localities: the Salanfe area and the Finhaut/Salvan area. The report will address the three main units of the Emosson area (metamorphic basement, Trias and Morcles nappe) in turn, outlining detailed observations and their resulting interpretations and implications. Since the Emosson area is dominated by the metamorphic basement much of the observations and interpretations are concerned with this unit.

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2. The Variscan metamorphic basement

The metamorphic basement present in Emosson is part of the Aiguilles Rouge massif and is the oldest and thickest unit found in the Emosson area. It is extremely complex and contains much folding and faulting. It is also a very variable unit containing nine formations in all.

Stratigraphy and rock descriptions

The GVS produced for the metamorphic basement is shown below (Fig. 2.1). The descriptions of all the metamorphic formations found within the GVS are presented after. The GVS for the metamorphic basement does contain a few uncertainties, which are as follows:

1. Thicknesses of the metamorphic rocks are only best estimate (the average of the two thicknesses taken from the cross section). 2. Thicknesses of the Tete des Ottans augen gniess Formation and Combe du Col mica schist Formation are unconstrained. 3. The GVS contains a non-scale gap where data cannot be accurately correlated. 4. Without the presence of younging data, the age relationships cannot be proven; here it is assumed that no overturning has taken place during the deformation history of the basement.

Page 15 Figure 2.1 – GVS to show the stratigraphy of the metamorphic basement

The Tete des Ottans augen gneiss Formation

Combe du Col mica schist Formation

Barrage de Barberine garnet schist Formation

Barrage de Barberine augen gneiss Formation

North of the big tunnel layered metasediment Formation North of the big tunnel garnet gneiss Formation

South of the small tunnel mica schist Formation North of the small tunnel layered metasediment Formation Small tunnel feldspar gneiss Formation

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The small tunnel feldspar gneiss Formation

Thickness – uncertain, approximately 100m

Colour – predominantly grey

Grain size – coarse grained

Outcrops – the small tunnel goes through this unit

Faulting – none observed

Approximate mineral percentages –

Mineral Percentages (%) Albite feldspar 40 Mica 30 Quartz 25 Hornblende and chlorite 5

This formation contains a high degree of milky white feldspar and a well- developed gneissose banding of light (quartz and feldspar rich) and dark (mica rich) bands. Some of the foliations are wavy due to the presence of alternating muscovite rich and poor layers (Fig. 2.2). This formation also contains shiny muscovite flakes, which can be seen inside the small tunnel. The small tunnel feldspar gneiss Formation appears to be much less eroded than other surrounding layers, this can be seen clearly when viewing the metamorphic basement from the other side of the lake.

Figure 2.2 - Notebook sketch showing wavy foliations or crenulations in the small tunnel feldspar gneiss Formation

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The north of the small tunnel layered metasediment Formation

Thickness – 65m

Colour – weathered orange in most places, fresh surfaces are predominantly grey

Grain size – medium-coarse grained

Outcrops – either side of the small tunnel

Faulting – contains many small high-angle faults

Approximate mineral percentages –

Mineral Percentages (%) Biotite 40 Muscovite 25 Quartz 20 Feldspar 10 Hornblende and chlorite 5

This formation is an intensely folded and faulted metamorphic rock with a schistose foliation. It contains a high proportion of flaky gold biotite and also contains quartz and muscovite. The rock outcrop at the type section consists of many layers, which are generally about 10cm thick (Fig. 2.3). Some of these layers are homogeneous and are not foliated. This formation also contains many quartz veins, most of which are about 50mm thick and thin out towards the ends; these run sub- parallel to the foliation. One of such quartz veins is boudinaged and others have been folded (Figs. 2.4 and 2.5).

Homogeneous ~10cm thick layers Figure 2.3 – Photograph of the north of the small tunnel layered metasediment Formation (grid ref 5603310273)

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Figure 2.4 – Notebook sketch of the fold geometry of a quartz vein just north of the small tunnel (grid ref 5603310273)

Figure 2.5 - Notebook sketch of the fold geometry of a quartz vein just south of the small tunnel (grid ref 5604210263)

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The south of small tunnel mica schist Formation

Thickness – 45m

Colour – predominantly grey, but has a purple tint south of the small tunnel

Grain size – medium-coarse grained

Outcrops – south of the small tunnel (where it has a purple tint) and north of the small tunnel

Faulting – many small faults

Approximate mineral percentages –

Mineral Percentage (%) Muscovite 50 Quartz 25 Biotite 10 Feldspar 10 Hornblende and chlorite 5

This formation is rich in large (2-3mm) silvery muscovite and also contains some biotite. It has a well-developed schistosity due to the alignment of the micas and in some instances the foliations are wavy due to the presence of muscovite rich and poor layers. This formation also contains many quartz veins, some of which are boudinaged (Fig. 2.6), and a blue-green homogenous layer that outcrops south of the small tunnel. Also present at this outcrop are folded quartz veins (Fig. 2.7).

Boudin neck

Foliations wrap around boudins Boudin

Figure 2.6 - Notebook sketch of boudinaged quartz veins (grid ref 5604710258)

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Figure 2.7 - Notebook sketch of the fold geometry of a quartz vein just south of the small tunnel (grid ref. 5604610262).

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The north of the big tunnel garnet gneiss Formation

Thickness – 150m

Colour – predominantly grey

Grain size – coarse

Outcrops – north of the big tunnel and north of the small tunnel

Faulting – is intensely faulted

Approximate mineral percentages –

Mineral Percentage (%) Muscovite 40 Garnet 20 Biotite 12 Quartz 12 Albite feldspar 12 Hornblende and chlorite 5

This north of the big tunnel garnet gneiss Formation contains alternating dark (mica rich) and light (quartz and feldspar) gneissose banded matrix (Fig. 2.8) with reddish almandine garnet porphyroblasts (~2mm in diameter) present uniformly on the foliation surface. This formation also contains sigmodial sheared quartz veins forming shear bands (Fig. 2.9). Some surfaces are rich in garnet porphyroblasts whilst other surfaces are sparse.

Figure 2.8 – Photograph of the north of the big tunnel garnet gneiss Formation

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Figure 2.9 – Notebook sketch of the shear bands (grid ref 5606810254)

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The north of the big tunnel layered metasediment Formation

Thickness – 40m

Colour – weathered orange in most places, fresh surfaces are predominantly grey

Grain size – medium-coarse

Outcrops – either side of the augen gneiss layer forming the synform

Faulting – contains many small-scale faults

Approximate mineral percentages –

Mineral Percentages (%) Biotite 40 Muscovite 25 Quartz 20 Feldspar 10 Hornblende and chlorite 5

This formation is an intensely folded and faulted metamorphic rock; it has weathered orange in places and has a schistose foliation. It contains a high proportion of flaky, gold biotite and also contains quartz and muscovite. The layers in the rock are generally about 10cm thick and some layers are not foliated. Many quartz veins are also present; these are usually sub-parallel to the foliation. One of these quartz veins is boudinaged (Fig. 2.10). Just north of the augen gneiss synform one of the quartz veins is folded (Fig. 2.11). Another folded quartz vein (Fig. 2.12) is present just north of the big tunnel.

Foliations

Boudin

Figure 2.10 – Notebook sketch of the boudinaged quartz vein

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Figure 2.11 - Notebook sketch of the fold geometry of a quartz vein just north of the augen gneiss synform (grid ref. 5598810369)

Figure 2.12 - Notebook sketch of the fold geometry of a quartz vein just north of the big tunnel (grid ref. 5607210253)

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The Barrage de Barberine augen gneiss Formation

Thickness – 310m

Colour – predominantly light grey, but chlorite can give it a dark blue-green tint

Grain size – course

Outcrops – makes up the large synform that can be tracked clearly from the west side of the lake

Faulting – contains many small-scale faults

Approximate mineral percentages –

Mineral Percentage (%) Albite feldspar 40 Quartz 20 Mica 20 Chlorite 15 Haematite? 5

This formation consists of a metamorphic rock of granite composition containing large (~10mm in diameter) milky white eye-shaped crystals of feldspar that run parallel to the foliation, consisting of alternating light (quartz and feldspar rich) and dark (mica rich) gneissose bands (Figs. 2.13 and 2.14). The layer is folded on the kilometre-scale (see structural observations in the metamorphic basement after the rock descriptions) and also contains folds on the metre scale (Fig. 2.15). Some areas of the rock are rich in ‘augens’ whilst other surfaces contain very few augens of feldspar (such as the outcrops near the carpark).

Alternating light and dark Eye shaped bands wrap feldspar around augens

Double eye Figure 2.13 - Notebook sketch of the Barrage de Barberine augen gneiss Formation (grid ref 5601210288)

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Eye-shaped feldspar

Figure 2.14 – Photograph of the Barrage de Barberine augen gneiss Formation

Figure 2.15 – Photograph of the metre- scale folding within the augen gneiss (grid ref 5600810294)

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The Barrage de Barberine garnet schist Formation

Thickness – 135m

Colour – predominantly grey

Grain size – medium-coarse

Outcrops – in the middle of the large augen gneiss synform

Faulting – intensely faulted, contains one fault which displays a flat and ramp geometry

Approximate mineral percentages -

Mineral Percentage (%) Muscovite 40 Garnet 20 Biotite 20 Quartz 10 Albite feldspar 10

This formation contains a well-developed schistosity due to the high content of aligned flaky mica (muscovite and biotite). It also contains reddish euhedral almandine garnet porphyroblasts (approximately 5mm in diameter); the garnets can be present uniformly on the foliation surface (Fig. 2.17) and the foliation can be seen to ‘wrap’ around the garnet (Fig. 2.16). The garnets found in this formation are slightly larger than the ones seen in the north of the big tunnel garnet gneiss Formation.

Foliation wraps around garnet Garnet

Figure 2.16 - Notebook sketch of one of the garnets showing foliation relationships, the garnet here is 4mm in diameter

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Garnets on foliation surface

Figure 2.17 - Photograph of a hand specimen of the Barrage de Barberine garnet schist Formation showing the foliation surface containing the garnets (divisions on the ruler are 10mm)

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Combe du Col mica schist Formation

Thickness – uncertain, approximately 50-100m

Colour – predominantly grey but has a green-blue tint

Grain size – medium-coarse

Outcrops – at the Combe du Col near the basement-Trias boundary

Approximate mineral percentages -

Mineral Percentage (%) Muscovite 35 Biotite 20 Quartz 20 Chlorite 12 Albite feldspar 12

This formation contains a well-developed foliation due to high content of large (2-3mm) silvery flakes of muscovite. It also contains elongate quartz veins that are parallel to the foliation.

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The Tete des Ottans augen gneiss Formation

Thickness – uncertain, approximately 500m

Colour – predominantly grey

Grain size – course

Outcrops – southwest of the house all the way up to the Col de Barberine and from the carpark to the top of Bel Oiseau.

Approximate mineral percentages –

Mineral Percentage (%) Albite feldspar 40 Quartz 20 Mica 20 Chlorite 10 Haematite 10

This formation is very similar to the Barrage de Barberine augen gneiss Formation except that the foliations are striking in a slightly different direction and the top of this unit is the unconformity surface where the eye-shaped feldspars are red/pink. This formation is not folded on the kilometre-scale like the other layer of augen gneiss but foliation readings show it is folded on the metre-scale. Some areas have very low concentrations of ‘augens’ and other areas have high concentrations of augens. In one outcrop the Tete des Ottans augen gneiss Formation contains resistant mineral veins running through it parallel to each other (Fig. 2.18).

Resistant mineral veins

Figure 2.18 – Photograph of an outcrop of the Tete des Ottans augen gneiss Formation containing resistant mineral veins (grid ref 5597510397)

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Structural observations within the metamorphic basement

This section encompasses more regional scale observations within the metamorphic basement, concentrating on observed folding and faulting.

Folding

The extent of the folding within the basement became apparent when viewed from the west side of Lac d’Emosson (Figs. 2.19 and 2.20). These pictures demonstrate that the Barrage de Barberine augen gneiss Formation is folded on the kilometre scale and that it shows a synformal structure.

Figure 2.19 - Photog raph of the synform taken from the west side of Lac d’Emosson and sketch of the photograph clearly showing the folded augen gniess unit. Numbers indicate where foliation measurements were taken, see Fig 2.21 for stereone t showing foliation readings)

Augen gneiss synform

Figure 2.20 – Photograph of another view of the augen gneiss synform, this time incorporating more of the surrounding metamorphic lithologies

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Strike and dip of foliations were taken from two separate localities on the synform: in the hinge of the fold and on one of the limbs (Fig. 2.19 shows localities). These measurements and their great circle of best fit have implications for determining the age relationship between folding and the formation of the foliations, the data can be presented on a stereonet (Fig. 2.21).

Figure 2.21 - Stereonet to show the relationship between foliations measured in the hinge and those measured in the limb of the Barrage de Barberine augen gneiss Formation synform (stereonets created using program created by R. Allmendinger)

The folded structure of the metamorphic basement can be best described via the cross section produced for the basement (Fig. 2.22). The process by which the cross section was determined and structure unravelled is outlined in interpretations of the metamorphic basement.

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Figure 2.22 - Cross section through the metamorphic basement showing its folded structure

Faulting

The metamorphic basement contains many small-scale high angle faults, which can be easily identified by the presence of very pronounced slickenlines on the fault plane surface. These slickenlines appear as small ridges or streaks on the surface of the fault plane and are often associated with mineral growth (mainly chlorite and quartz). They occur packed densely together in fibrous sheets or bunches in which the fibres are strongly aligned; and they are commonly sub-parallel to the fault plane. The slickenlines observed contained a distinct stepped structure (Fig. 2.23). The stepped structure of the slickenlines can be used to determine which side of the fault has slipped. Fig. 2.23 can be modified to show the footwall and hanging wall (Fig. 2.24).

Stepped slickenline structure on fault surface

Figure 2.23 - Sketch showing stepped slickenline structure (side view)

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Figure 2.24 - Sketch showing how the stepped structure of the slickenlines can show which side is the footwall and which is the hanging wall

Much of the small-scale faulting is present in the Barrage de Barberine garnet schist Formation; this formation also contains a fault with a stepped geometry containing flats and jogs (Fig. 2.25).

Slickenlines

Flats or straights

Jogs or ramps

Figure 2.25 - Notebook sketch of the stepped geometry of one of the faults (grid ref 5600310343)

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These small-scale faults typically only have displacements of at most a few metres. However, the metamorphic basement also contains a large fault that has led to the displacement of most of the folded metamorphic formations (Fig. 2.26).

Fault trace

Displaced metamorp- hic formations

Figure 2.26 - Part of the final map showing the position of the large fault that displaces many of the metamorphic formations

The strike and dip of the fault plane, pitch of the slickenlines, minerals present on the slickenlines and which side fell were measured for 140 of the small-scale faults present in the basement. The strike orientations for the 140 fault planes can be plotted onto a rose diagram to find the average strike orientation (Fig. 2.27).

Figure 2.27 - Rose diagram to show the orientations of the 140 measured fault planes (green line indicates average orientation)

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The strike and dip of the fault plane and the pitch of the slickenlines for the 140 measured faults can be plotted onto a stereonet (Fig. 2.28). Although this volume of data would not normally plotted as planes it has implications for the calculation of the orientation of principal stresses.

Figure 2.28 - Stereonet to showing the measured fault planes and slickenlines for the 140 measured faults

Other observations

An AFM diagram has been constructed to show approximate mineral assemblages for the garnet pelites in the Emosson area, assuming that the sillimanite found with the garnets in the west is present on the east (Fig. 2.29).

Figure 2.29 - AFM diagram to show approximate mineral assemblages for Emosson pelites

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Interpretations of the metamorphic basement

The Alpine orogeny had comparatively little effect on the metamorphic rocks seen in the Emosson area since they show low-grade metamorphism; therefore pre- Alpine structures and mineral assemblages can be well preserved (Pilloud, 1991; Dobmeier and von Raumer, 1995). Such preservation may mean that foliations found within the metamorphic rocks could relate to previous bedding or laminations of sedimentary rocks. Prior to metamorphism the rock types would have probably consisted of a succession of shales and mudstones.

Metamorphic grade

With the lack of grade-determining minerals (apart from sillimanite seen on the western side) calculating accurate PT conditions for basement metamorphism proved difficult. Von Raumer and Schwander (1985) studied the chemical evolution of garnets and garnet zoning in the Emosson area in order to constrain PT conditions. They found that the metapelites yielded an early pressure climax, 10kbar and 500- 550ºC, which was followed by a thermal peak at 4-6kbar and 650-700ºC. The conditions calculated by von Raumer and Schwander (1985) correspond to greenschist or possibly amphibolite metamorphism (Fig. 2.30), the term sub- greenschist facies is more applicable to Emosson since it refers to low-grade metamorphic rocks collectively. The neighbouring Mont Blanc massif experienced different metamorphic conditions during the Carboniferous and Permian periods due to differing uplift rates and geothermal gradients (Marshall et al 1997).

Generalised position of the metamorphic rocks of Emosson in PT space

Figure 2.30 - PT diagram showing the positions of greenschist and amphibolite facies (from Winter 2001)

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Formation of augen gneiss

Scotford (1969) investigated augen gneiss within greenschist facies in western Turkey: a similar situation to that seen in Emosson. He found that structural, stratigraphic, or intrusive explanations of the occurrence of the augen gneiss, a rock typically only associated with only the highest grade metamorphic rocks, in such a low-grade environment were unsatisfactory. He stated that petrographic evidence of the replacement of sodic plagioclase by potassium feldspar at the borders of the body points to a metasomatic origin in which the surrounding phyllites and schists have been converted to augen gneiss locally through the effect of ascending fluids. The resistant mineral veins in the Tete des Ottans augen gneiss Formation could have been formed as a result of high temperature fluids travelling along cracks or fractures within the rock during its formation (Fig. 2.18). He estimated the temperature environment for this metasomatic event to be in the range of 500 ± 50°C from the structural state of the potassium feldspar augen and the mineral paragenesis of gneiss and associated phyllites and schists. This temperature range lies within the 400-500°C range for greenschist metamorphism; therefore the augen gneiss can be produced by that process in an environment encompassed by the greenschist facies. The eye-shaped feldspars in the augen gneiss (Fig. 2.13) probably reflect large feldspar crystals in the parent rock type. The foliation present in the augen gneiss wraps around the augens suggesting that they were present before the main foliation formed.

Different formations of augen gneiss

The cross section (Fig. 2.22) for the metamorphic basement shows that Barrage de Barberine augen gneiss Formation is stratigraphically not in contact with the metamorphic-Trias boundary. Since augen gneiss can be seen at the boundary and the top of such augen gneiss shows weathering due to the presence of the unconformity then there must be more than just one formation of augen gniess. From mapping alone the presence of two formations is not clear, since the augen gneiss in the area is very similar in appearance. Another method to determine the location of different formations must therefore be used. The folded augen gneiss synform (Fig. 2.19) plunges at a high angle (see stereonet 2.21), this angle is steep enough for the fold to plunge over the topography currently present on the west side of Lac d’Emosson. Therefore, the augen gneiss found on the west side during the reconnaissance cannot be the same formation as the augen gneiss making up the synform seen on the east. This observed augen gneiss on the west appears to trace over to the augen gneiss present next to the unconformity on the east side (Figs. 2.31 and 2.32).

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Augen gneiss that visibly traces across the lake to the east side Augen gneiss that plunges over the topography present on the west side

Figure 2.31 - Part of the 1:25000 reconnaissance map specifically showing how the augen gneiss on the west side of Lac d'Emosson joins up with that seen on the east

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Figure 2.32 - Photograph of the augen gneiss on the west side of Lac d'Emosson, the picture also shows the Trias, Morcles nappe and their boundaries. The second picture is coloured and labelled to clearly identify the separate units. The augen gneiss shown here traces across the lake to the augen gneiss next to the unconformity on the east side.

Stereonet data shown in Fig. 2.33 shows that the strike and dips of the foliations of the augen gneiss on either side of the lake are similar, further suggesting they are related. This evidence suggests that there are two individual formations of augen gneiss; this is shown on the GVS produced for the metamorphic basement (Fig. 2.1).

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Figure 2.33 - Stereonet to show the relationship between the foliations of the augen gneiss on the west side next to the unconformity and the augen gneiss on the east next to the unconformity

Folding

One of the main objectives when studying the metamorphic basement was to understand its complex folded structure. On the basis of the large augen gneiss synform seen from the west (Figs. 2.19 and 2.20), the other metamorphic rocks were investigated to see if they also fit into a larger folded structure. Folds characteristically develop simultaneously at different scales; so large folds include smaller-scale folds in their limbs and hinge zones. Therefore, by interpreting the geometries of small-scale folded quartz veins within the different metamorphic formations their position in terms of large-scale kilometre folding can be found. The positions of where these small fold geometries can be found within a larger folded structure are shown in Fig. 2.34.

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Z-folds found on the ‘rising’ limb S-folds found on the

‘falling’ limb

Figure 2.34 - diagram showing the different orders of folds and where s and z folds can be found (modified from Twiss and Moores 2000)

The small fold geometries are classed into two categories based on whether the short limb has been rotated clockwise (Z-folds) or counterclockwise (S-folds). Both categories of fold geometry are present in the metamorphic basement (Fig. 2.4 shows a Z-fold and Figs. 2.5, 2.7, 2.11 and 2.12 show S-folds). Using this information and the presence of repeating lithologies found during detailed mapping the structure of the metamorphic basement could be interpreted and the cross section produced. Part of the cross section has been reproduced to show the location of the S- and Z- folds found within the basement (Fig. 2.35). The location of the S- and Z-folds in the basement corresponds to predicted positions shown in Fig. 2.34.

Figure 2.35 – Part of the resultant cross section for the metamorphic basement specifically showing the location of the s and z fold geometries (s-folds denoted by S and z-folds by Z)

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With the presence of pronounced foliations throughout the metamorphic basement it was logical to determine whether these foliations have been folded by the large-scale kilometre folding or whether they developed after the folding had occurred. The foliation measurements collected from the two localities shown in Fig 2.19 line up onto a great circle and are therefore folded (Fig. 2.21); hence the foliations were formed before the folding ensued. However, the data is inconclusive when calculating the precise hinge line of the large fold since there are a number of possible positions that the best-fit great circle could occupy. From the data is it only possible to state that the synform plunges steeply or sub-vertically. The evolution of this km-scale folding is more complex than just one homogeneous deformation event producing a symmetrical fold however. The synform plunges steeply (shown in stereonet, Fig. 2.21) and the antiform, which forms the mountain Bel Oiseau, can be traced across the lake to the west side and therefore must plunge sub-horizontally. This information is shown in Fig. 2.36 and proves that there could have been at least two stages of deformation and the very heterogeneous distribution of stress during folding.

Figure 2.36 – Schematic 3D model to show the nature of the metamorphic basement km-scale folding, the photograph on the right also shows the folding in 3D using a piece of card

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Faulting

The slickenlines on the surface of the fault plane are mineral fiber lineations that are formed by elongate crystal grains of a particular mineral. The structure and composition of the mineral fibers are so distinct from those of the rock in which they occur that it is clear the fibers have grown as the faulting occurred. Some of the mineral fibers are curved or stepped indicating a change in the fault orientation during deformation or possible fault reactivation. Nearly all of the mineral fiber lineations on the small-scale faults contain both quartz and chlorite, this suggests that the formation of these faults preceded most of the metamorphism experienced by the area. Von Raumer and Schwander (1985) stated that the area underwent greenschist and even approached amphibolite metamorphism; such metamorphism would have produced mineral fiber lineations containing higher-grade minerals. Fig. 2.27 shows the orientations of the faults measured in the basement, it can be seen from the rose diagram that the faults have a strong orientation preference; this may suggest that they have been formed by the same or similar processes. Using the program FaultKin created by R. Allmendinger (2001) the fault plane solution can be determined for the 140 faults measured in the basement (Fig. 2.37).

Figure 2.37 - Stereonet showing fault planes and calculated fault plane solution, smaller stereonet to the right shows the fault plane solution more clearly (black = compression, white = dilatation)

The fault plane solution or focal mechanism shows a dip-slip dominance with some strike-slip. This fault plane solution can then be used to calculate the orientation of the principal orthogonal stresses (σ1, σ2, σ3). σ1 is the largest principal stress, σ3 is the smallest and σ2 is the intermediate principal stress. σ2 is given by the intersection

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of the nodal planes and so can be well constrained, σ1 and σ3 are less well constrained but must exist on the plane at 90˚ to σ2. Uncertainty arises in attempting to relate the P and T axes (Fig. 2.38) with the greatest and least compressive stress directions. This would be correct only if the stress-drop tensor is the same as the stress tensor, which is unlikely ever to be the case; this is because the stress-drop tensor contains only shear stresses. The P and T axes are at 45° to the fault plane, which is not the correct direction for Coulomb failure (in reality it is usually ~30º implying that the results obtained could have an error in the region of 15º).

Figure 2.38 - Geometry of the double- couple earthquake fault plane solution showing positions of P and T axes

Since the fault is a pre-existing plane of weakness, the ambiguity could be greater than that; since there is a wide range of possible orientations of σ1 and σ3 with respect to the fault plane, depending on the friction value assumed (McKenzie, 1969). However, since the metamorphic basement underwent subgreenschist metamorphism with temperatures mainly between 500-550ºC (von Raumer and Schwander, 1985) the rock would have been fairly ductile; if this were true then the recorded palaeostress for the Emosson area would be closer to the true value. This is because the failure envelope for ductile materials is much different than that for brittle materials, consequently the 2θ angle defined by the Mohr circle would be much closer to 45º (Fig. 2.39):

Figure 2.39 - Mohr diagram showing the failure envelopes for brittle and ductile materials and their 2θ angles

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This would mean that the palaeostress results for the Emosson area are much more accurate than first thought. Diagrams used to estimate positions of σ1 and σ3 are as follows (Fig. 2.40):

Figure 2.40 - Stereonets to show the approximate positions of σ1, σ2, and σ3. The middle of the red contour lines defines the position of σ1 and the middle of the blue contour lines defines the position of σ3

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Different methods of palaeostress analysis are based on different assumptions and use very different approximations, and so could possibly yield different results. Liesa and Lisle (2002) addressed the applicability of palaeostress analysis from fault data, they stated that the researcher will always be required to take some part in the correct choice of the tensors, and additional structural data (stylolites, tension gashes) will be very useful for the correct separation of stress tensors from fault-slip data. Despite the assumptions and approximations the palaeostress analysis and resultant plunge and plunge directions of principal stresses produced for the Emosson area provide a good first approximation (Fig. 2.41):

Figure 2.41 - Stereonet to show the positions of the three principal stresses (σ1, σ2 and σ3)

With this information a schematic cross section showing the fault plane, axial plane and principal stress orientations can be constructed (Fig. 2.42):

Figure 2.42 - Schematic cross section showing principal stress orientations

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The direction of σ1 could be related to the resultant force of compressive and extensional forces in an evolving mountainbelt (Fig. 2.43):

Figure 2.43 - Cross section showing possible source of σ1

Implication of the garnets in metamorphic formations

Two of the formations within the metamorphic basement contain garnets (Barrage de Barberine garnet schist Formation and the north of the big tunnel garnet gneiss Formation), the presence of the garnets does not relate to different PT conditions experienced by the rocks but different starting concentrations that have favoured the formation of garnet. The metapelites containing the garnets are more CaO-rich, whilst in the other metapelites (CaO-poor) the garnets are absent (von Raumer and Schwander 1985); thus the concentration of CaO determined the presence or absence of garnet. Even within the garnet-rich layers there are areas of high garnet density and low garnet density, this must be due to CaO concentration differences within the formation itself.

Other interpretations

The top of the Tete des Ottans augen gneiss Formation contains red/pink feldspars; this is due to weathering during the period of non-deposition and demonstrates the high haematite content of the augen gneiss. The boudins found in the metamorphic basement (Figs. 2.6 and 2.10) were formed by the pulling apart of a linear layer along periodically spaced lines of separation called boudin lines. Observations stated that some quartz veins are boudinaged, however von Raumer and Schwander (1985) found that metabasic rocks are found within the boudins in the metapelites in the Emosson area.

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The sheared quartz veins in the north of the big tunnel garnet gneiss Formation (Fig. 2.9) were formed by the following shear model (Fig. 2.44):

Sigmodial shaped Direction of quartz vein shear stress

Figure 2.44 - Notebook sketch of the model of shearing for the sigmodial sheared quartz veins

More regional shear models cannot be interpreted with such a small sample data; this interpretation has been included for completeness.

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3. The granite

The end of the metamorphic basement to the south is represented by a large body of granite, which makes up the mountain Six Jeur (2039m).

The Chapelle d’Emosson white granite Formation

Stratigraphy and rock descriptions

Since the age relationship between the granite and the metamorphic basement could not be determined it has been excluded from the main body of the GVS. The granite could have been emplaced at any time during the evolution of the region, hence its stratigraphy cannot be explained. The rock descriptions for the Chapelle d’Emosson white granite Formation are presented below:

Thickness – unknown due to poor accessibility, may extend as far as the town of Vallorcine

Outcrops – on Six Jeur (2039m), near the Chapelle d’Emosson

Grain size – coarse, very coarse in places (some grains 40mm)

Colour – predominantly white, but mottled by dark biotite and hornblende

Faulting – some small-scale faults are present

Folding – none

Approximate mineral percentages –

Mineral Percentage (%) Quartz 65 Alkali feldspar 23 Plagioclase feldspar 18 Biotite and hornblende 10

The Chapelle d’Emosson white granite Formation is an acidic granitic rock with a high overall silica content (Fig. 3.1). It contains euhedral milky feldspars ranging from 2-30mm, which are dominantly alkali feldspar. The quartz present is predominantly anhedral and grains range from 2-40mm.

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Figure 3.1 - hand specimen of the Chapelle d’Emosson white granite Formation

The calculated mineral compositions can be plotted on a QAP diagram (Fig. 3.2):

Figure 3.2 - QAP diagram showing the position of the Chapelle d’Emosson white granite Formation, shown as the red point

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Interpretations of the granite

Using the QAP diagram (Fig. 3.2) the Chapelle d’Emosson white granite Formation has been determined as a quartz-rich granitoid. The extent of the granite is unclear but it is sufficiently large to be classed as a batholith rather than as just a sill or dyke. Estimating the age of the granite in relation to the metamorphic basement from field data proved an impossible task since the timing of batholithic activity relative to deformation in an orogen can be very variable. The granite-metamorphic contact showed that there is no increase in metamorphic grain size close to the contact; this is due to the already large size of the grains in the metamorphic rock. A large grain size means that there is a long diffusion path for minerals to travel in order to form new or larger grains. In the literature the granite body which makes up Six Jeur is referred to as the Vallorcine granite, and Brandlein et al (1994) found that the Vallorcine granite intruded the pre-Mesozoic basement of the Aiguilles Rouges massif during the late Variscan, and therefore suffered no Variscan metamorphism. Folding and faulting observed within the basement is not present in the granite, this also suggests emplacement at a later period. Brandlein et al (1994) stated that its emplacement was accompanied by strike slip tectonics and also found that there is a main body of granite about 15km long and a lower, more coarse-grained porphyritic granite can be differentiated from the main body. The main granite facies corresponds to an S-type granite; its formation includes the magmatic ascent at adiabatic conditions after fusion under low water activities and internal differentiation during the cooling stage (Brandlein et al, 1994).

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4. The Trias

The Trias is an autochthonous layer of sediment that unconformably overlies the metamorphic basement. The Trias consists of three major rock formations: the signpost sandstone Formation, the signpost mudstone Formation and the Col de Barberine cargneule Formation.

The signpost sandstone Formation

Stratigraphy and rock descriptions

The log GVS for the sandstone and mudstone is presented below (Fig. 4.1) showing the formations and their members. The signpost sandstone Formation rock descriptions follow the log GVS.

Page 54 Angular polygonal The Col du hollows Barberine cargneule Formation

The signpost mudstone Gently dipping Formation faint laminations

Non-scale gap

Thin layers of sandstone

The Col du Barberine interbedded sandstone and mudstone Member Green mudstone elongate clasts

Gently dipping The signpost faint laminations sandstone Formation (numbers indicate different beds)

Unconformable top of metamorphic basement Figure 4.1 - log GVS for the sandstone and mudstone formations and their members Page 55

Thickness – total thickness measured at 310cm, 306cm and 301cm at three different locations

Bedding – there are 5 different beds of sandstone; which are 64cm, 56cm, 84cm, 48cm and 49cm thick respectively (from oldest to youngest). The beds are also fairly homogenous.

Colour – generally creamy-white, however one outcrop is purple in colour

Grain size – coarse grained

Roundness – well rounded

Sorting – fairly well sorted, although does contain a few pebble sized grains

Faunal content – dinosaur footprints well preserved on the top of the youngest sandstone bed. A possible brachiopod is present on second oldest bed.

Sphericity – coarse sand grains are well rounded; pebble sized grains are subangular

Sedimentary structures – contains straight crested, symmetrical ripples (Fig 4.2) and gently dipping faint cross laminations

Textural maturity – very mature, sandstone contains very high percentage of quartz

Folding – none

Faulting – contains several mappable faults

Figure 4.2 – symmetrical ripples as seen on the top of one of the sandstone beds (grid red 5595610433)

Just below the signpost sandstone Formation is a thin (about 20cm thick) layer of matrix supported quartz conglomerate. This layer is present in situ in Salanfe but is only present in loose fragments on the east side of Lac d’Emosson. It contains large pink and purple fist-sized quartz crystals within a coarse sand grained matrix (Fig. 4.3). The quartz conglomerate does not appear on the GVS for the Trias since it was

Page 56 not seen in situ on the east side of Lac d’Emosson (see Fig. 1.7 for stratigraphic position of the layer).

Large purple quartz clast

Figure 4.3 – Photograph of the quartz conglomerate containing fist-sized quartz grains found on broken rock boulder (taken on the east side of Lac d’Emosson, grid ref 5595510427)

Strike and dip of bedding throughout the sandstone and the mudstone measured from numerous localities throughout the Emosson area can be plotted on a stereonet (Fig. 4.4).

Figure 4.4 - Stereonet to show the relationship of the sandstone and mudstone measured in different localities

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Percentage composition estimates for the sandstone are represented graphically in Fig. 4.5.

Figure 4.5 – Pettijohn classification of sandstones diagram, composition of the signpost sandstone Formation shown as red coloured region

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Dinosaur footprints

The dinosaur footprints are well documented on the west side of Lac d’Emosson, especially in the Veiux Emosson area (Fig. 4.6), but have never been acknowledged on the east side. The dinosaur footprints shown in Figs. 4.7, 4.8 and 4.9 are present on a sandstone bed on the east side of Lac d’Emosson, close to where the path splits into two (grid ref 5595414033). The dinosaur footprints can be recognised by the presence of mudstone in the footprint itself.

Figure 4.6 - Photograph of the dinosaur footprints seen in the Vieux-Emosson area (grid ref 5571509984)

Dinosaur footprint one (Fig. 4.7)

Number of toes – three

Depth of footprint – 25mm

Other observations - the middle toe is longer (60mm) than the other two toes (both 45mm)

Figure 4.7 - dinosaur footprint one discovered on the east with interpreted drawing

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Dinosaur footprint two (Fig. 4.8)

Number of toes – four

Depth of footprint – 25mm

Other observations – contains two larger toes in the middle (60mm and 70mm) and two smaller toes at the edge of the foot.

Figure 4.8 - dinosaur footprint two discovered on the east with interpreted drawing

Dinosaur footprint three (Fig. 4.9)

Number of toes – uncertain, however a similar footprint in the Vieux-Emosson area contained seven small toes.

Depth of footprint – 55mm

Other observations – none

Figure 4.9 - dinosaur footprint three discovered on the east with interpreted drawing

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The Col de Barberine interbedded mudstone and sandstone Member

Directly above the signpost sandstone Formation is the Col de Barberine interbedded mudstone and sandstone Member: the transition from the sandstone to the above mudstone layer This unit contains interbedded layers of the signpost sandstone Formation described above and the signpost mudstone Formation described in the next section. The rock descriptions, features, textures etc. are therefore the same for this unit.

Thickness – approximately 155cm (inaccurate due to non-scale gap)

Sedimentary structures – contains discontinuous, thin (~3mm) starved sandstone ripples and faint cross laminations within mudstone layers. This member also contains small (2mm thick) layers of continuous cream sandstone.

Extra features – the thickest unit (82cm) at the base of the member contains elongate clasts of green mudstone, which are about 50mm in diameter

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Interpretation of the sandstone features

The signpost sandstone Formation is a very mature sandstone which contains a very high percentage of quartz with little feldspar and lithics. Plotting the composition on the diagram shown in Fig. 4.5 it can be seen that the rock type is a quartz arenite.

Symmetrical ripples

The symmetrical, straight crested ripples seen in the sandstone are wave ripples (Fig. 4.2) and were hence formed by a standing body of water. Ladderback ripples are also present; these are due to a change in the direction of water movement over an area of ripples producing a secondary set of ripples within troughs of larger ripples. Other than general observations the ripples were tested using the ripple index to help distinguish what type of ripples they are. The ripple index is given by:

Ripple index = L (Fig. 4.10) H

Figure 4.10 - calculating the ripple index

For the ripples here the ripple index was calculated as 4.6, since current ripples have a ripple index of >5 then the ripples here must be wave-formed. In the reconnaissance on the east side of Lac d’Emosson current ripples and wave ripples were seen on the same sandstone bed, this indicates the interaction of number of different processes simultaneuously.

The conglomerate layer

The thin layer of matrix supported quartz conglomerate found just below the sandstone (Fig. 4.3) is probably the result of a terrestrial mud slide or debris flow. Here the strength of the sand matrix is most likely to be the clast-supporting mechanism; these types of debris/mud flows are termed cohesive and are common in subaqueous settings. The debris flow is very laterally consistant and is present in situ in Salanfe area one. Cohesive debris flows tend to de-water slowly which sustains their mobility and are therefore able to transport sediment long distances and spread laterally to give extensive sheets.

Folding within the sandstone and mudstone

A stereonet was plotted in order to check that the Triassic sandstones and mudstones are folded (Fig. 4.4), the stereonet shows that the strike and dip of the bedding forms a tight cluster and therefore the mudstone and sandstone are not folded.

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However, this does not mirrior regional trends since the sandstone and mudstone are folded in the Vieux-Emosson area near the dinosaur footprints (grid ref 5571509984).

Dinosaur footprints

The presence of many dinosaur footprints on one of the sandstone beds suggests that the area has been frequented by numerous animals. For the excellent preservation of the dinosaur footprints there must have been the occasional rapid hardening of sediment promoting drying following more wet periods. This may provide evidence for palaeoenvironment which is discussed in the section: palaeoenvironmental anaysis of the Trias. Dinosaur footprints one and two (Figs. 4.7 and 4.8) both have depths of 25mm, this could signify that the dinosaurs have similar body masses. The dinosaur that made footprint three (Fig. 4.9) must have been much heavier since footprint three is 55mm deep. The toes on print two are well defined and the rest of the footprint is poorly preserved, this could imply that the dinosaur walked with most of its weight on its toes. An interpretation of what the dinosaurs could have looked like is shown in Fig. 4.11.

Figure 4.11 - Impression of what the dinosaurs in the area could have looked like (Museum de Geneve 2003)

The interbedded member

The Col du Barberine interbedded sandstone and mudstone Member between the sandstone and the mudstone probably marks the transition between their two different depositional environments. One of the sandstone beds within the interbedded formation contains elongate clasts of green mudstone, these could be due to the re- working of eroded mudstone nodules.

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The signpost mudstone formation

Stratigraphy and rock descriptions

The stratigraphy of the mudstone is explained in the GVS shown in Fig. 4.1.

Thickness – approximately 238cm (inaccurate due to non-scale gap)

Bedding – contains many small (~10cm) beds

Colour – beds are either green or purple (Fig. 4.12)

Grain size – very fine grained

Roundness – N/A

Sorting – very well sorted

Faunal content – none, beds are unbioturbated

Sphericity – N/A

Sedimentary structures – faint cross laminations, well-preserved symmetrical ripples, discontinuous sandstone layers (Figs. 4.12 and 4.14) and occasionally a small (~2mm thick) cream sandstone layer.

Textural maturity – N/A

Folding – none

Faulting – contains several mappable faults

Fig. 4.12 shows some of the distinguishing features of the signpost mudstone Formation and Fig. 4.13 shows its position relative to the signpost sandstone Formation.

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Purple mudstone

Small sandstone layer

Green mudstone

Discontinuous sandstone layer Figure 4.12 – Photograph the signpost mudstone Formation showing some of its distinguishing features

Figure 4.13 - Photo showing the relative positions of green mudstone, red mudstone and the sandstone. The copy on the right shows the position of the units (grid ref 5595710433)

Figure 4.14 - Photograph showing a thin layer of sandstone draped over the mudstone ripples, in cross section they appear as discontinuous layers of sandstone (grid ref 5595710433)

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Interpretation of the mudstone features

The two types of mudstone

The different types of mudstone (purple and green mudstone) are due to the different forms of iron present in the rock and do not reflect different parent compositions. The green mudstone is the less oxidised form and contains FeO (ferrous iron compounds) while the purple mudstone is more oxidised and contains Fe2O3 (ferric iron compounds). So, in general, the area experienced a period of high oxidation followed by a period of low oxidation (Fig 4.13 shows green mudstone overlying purple mudstone). Waterlogging of sediment, where water occupies the space between grains usually occupied by air, usually produces anoxic or less oxidised conditions. The green mudstone is therefore likely to have been deposited during waterlogged conditions when the water table was high, and the purple mudstone deposited in drier, well-aerated conditions when the water table was low. The expectation would be that the anoxic conditions would be produced at greater depths, placing the green mudstone below the purple mudstone. However, this is not the case and so the presence of the different types of mudstone must be due to a period of low water level followed by a period of high water level. This is only a general trend however since some areas experienced local variations (Fig. 4.12 shows red mudstone overlying green mudstone) and others contained alternating green and purple layers.

Ripples and other features

The well-preserved symmetrical ripples are wave ripples similar to those seen in the sandstone and therefore were also formed in a standing body of water. The discontinuous layers of sandstone are starved sandstone ripples, these form when there is not enough sandstone present to form ripples in a ripple-forming regime. Fig. 4.14 shows the starved sandstone ripples in the troughs of mudstone ripples. The mudstone is very fine grained and must have been formed in a low energy environment; palaeoenvironment of the mudstone and interpretations of other features used to interpret palaeoenvironment are discussed in the palaeoenvironmental anaysis of the Trias section.

.

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The Col du Barberine cargneule Formation

Stratigraphy and rock descriptions

The log GVS for the cargneule, showing its relative position to the mudstone and the carbonate nappe, is presented below (Fig. 4.15). The formation, and its members, are described afterwards.

Figure 4.15 – log GVS for the cargneule (continued at the bottom of the Page 67 opposite log)

Combe des Fonds carbonate nappe Formation

Thrust surface between carbonate nappe and cargneule

The river valley white structureless Member

The river mouth quartz limestone Member

The Col du Barberine cargneule Formation (numbers indicate different beds)

Top of the Log continued here signpost mudstone Formation

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Thickness – approximately 20m thick; measuring thickness proved difficult due to outcrop patterns and the several faults which were present in the cargneule.

Bedding – appears to be bedded in some places (proved difficult to measure) but it is mainly massive

Colour – cream-yellow to brown in places

Grain size of matrix – fine-grained matrix

Matrix composition – carbonate

Clast or matrix supported – matrix supported

Clast size – ranges from 5-200mm in diameter

Clast origin – mostly intraformational (clasts derived from surrounding rocks)

Variety of clasts – cargneyle is a polymictic conglomerate

Clast lithologies – quartz, mudstones and other carbonate rock

Distribution of clasts – heterogeneously distributed

Faunal content – none

Folding – contains a large (10m scale) fold (Fig. 4.16) and a small (10cm scale) fold, the cargneule is the only Triassic unit to display evidence for deformation

Faulting – an inferred fault runs up the river valley straight through the middle of the cargneyle; also contains smaller faults which displace units within the cargneyle

Sedimentary structures – contains polygonal angular hollows which give the rock a ‘crunchie bar’ appearance

The cargneule can be split into two different types (excluding the different members): one that contains angular polygonal hollows (Figs. 4.18 and 4.19) and another that contains angular carbonate, quartz and mudstone clasts (Fig. 4.20).

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Figure 4.16 – Photograph of fold in the cargneyle, the second picture has superimposed lines to show the folding more clearly (grid ref 5597010477)

A stereonet using folded bedding measurements from the fold in Fig. 4.16 is shown below (Fig. 4.17):

Figure 4.17 - Stereonet to show the bedding measurements taken from the fold within the cargneule

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Polygonal angular hollows giving a ‘webbed’ appearance

Figure 4.18 – Hand specimen of the type one cargneule containing angular polygonal hollows

Figure 4.19 - Type one cargneule showing webbed 'crunchie bar' appearance

Carbonate clast

Figure 4.20 – Hand specimen of the type two cargneule containing carbonate clasts

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Type one is present at the base of the cargneule and type two is present above this, exact boundaries between the two types are difficult to distinguish due to the complex nature of the cargneule and the numerous faults that run through it. Within the cargneule unit there are two Members: the river valley white structureless Member (Fig. 4.21) and the river mouth quartz limestone Member (Fig. 4.22), both are described below:

The river valley white structureless Member

Position within the cargneule – 617cm from the base of the cargneule

Thickness – 330cm

Colour – brilliant white

Bedding – contains three beds which are 110cm, 100cm and 120cm thick respectively (oldest to youngest)

Grain size – very fine grained

Faunal content – none

Sedimentary structures – none

Folding – none

Faulting – is displaced by small faults with a few metres displacement

This unit is much more extensive on the west side of Lac d’Emosson where it is seen as a mappable formation.

Figure 4.21 - Hand specimen of the river valley white structureless Member

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The river mouth quartz limestone Member

Position within the cargneule – directly above the cargneule

Thickness – about 200cm

Colour – pale blue/grey

Bedding – contains small (~30mm thick) layers

Grain size – fine grained

Faunal content – none

Sedimentary structures – none

Folding – small layers are folded and also look deformed (Fig. 4.22)

Faulting - none

Figure 4.22 – Photograph of the river mouth quartz limestone Member showing small folded layers (grid ref 5594410479)

The river mouth quartz limestone Formation contains a large number of quartz and possibly calcite veins.

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Interpretation of the cargneule features

Despite the absence of observed dolomite it is commonly referenced in association with cargneule in much of the literature and most carbonates in evapouritic basins are highly altered and dolomitised. Cargneule is a non-dolomitic veined or brecciated rock, usually yellow-brown in colour, associated with Trias dolomite, gypsum and anhydrite (Warrak, 1974; Grandjacquet & Haccard, 1975).

The two types of cargneule

The cargneule is a very complex unit and proved difficult to understand. The angular polygonal hollows found within the type one cargneule (Figs. 4.18 and 4.19) are likely to be hollows once occupied by halite, since they are mainly cubic in shape, and have subsequently been eroded or dissolved away. Some of the clasts in the type two cargneule (Fig. 4.20) are very similar in appearance to the mudstone seen in the area, these are described as intraformational clasts and have probably been liberated by desiccation of the pre-existing mudstone along the shoreline and incorporated in the cargneule before reworking. Other clasts seen (quartz clasts) could also have been incorporated in this way. The presence of mudstone in the cargneule suggests that the mudstone must have been lithified prior to the formation of the cargneule.

Proposed mechanisms for the formation of cargneule

A variety of mechanisms have been proposed for the formation of cargneule. Masson (1972) proposed that cargneule forms by hydraulic fracture, with a later infilling of calcite, however Warrak (1974) showed that calcite cannot be regarded as vein-filling. Wilson (1978) considered the cargneule from the Theodal-Rothorn zone (Zermatt, Switzerland) to originate from dolomite and calcite limestones in high strain zones, and cargneule formation occurred with an increase in strain. Wilson (1978) stated that diffusive processes then altered the cargneule and that the gypsum is absent and is not significant. Warrak (1974) studied cargneule in the Frejus region of the French Alps and suggested that cargneule is the product of dolomite and gypsum after a combination of processes including fracturing, veining brecciation, replacement of gypsum by calcite, dolomitisation of vein material and dedolomitisation has occurred. He stated that the conversion to anhydrite to gypsum could be responsible for the brecciation. Vearncombe (1982), who considered these different processes, modified those proposed by Warrak (1974) and suggested there are three main elements to cargneule formation: the conversion of dolomite to calcite (dedolomitisation), brecciation and the local inclusion of other material. Amieux and Jeanbourquin (1989) actually studied the cargneules in the Valais region and proposed nine diagenetic stages for its development. They stated that each diagenetic stage begins with several types of fracturing or dissolution, continues at times with internal sedimentation and ends with a single phase of cementation, which characterises the stage of formation.

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Deformation in the cargneule

The cargneule is the only part of the Trias that shows evidence for deformation (Fig. 4.16). This is due to the fact that the top of the cargneule is the thrusting or decollement surface that the Morcles nappe was emplaced on. Cargneule and associated dolomite, anhydrite and gypsum are often connected with the soles of thrust planes and the gypsum has been regarded as a significant decollement horizon in both the low grade metamorphic external and high grade internal zones of the Western Alps (Ramsay, 1963). It is suggested that dolomite + anhydrite + water can act as an important decollement horizon. The fold within the cargneule (Fig. 4.17) was measured in a similar way to the folds measured in the Morcles nappe to check relatedness and is shown on the stereonet in Fig.7. The hinge line produced for the fold in the cargneule is similar to those measured in the nappe (Fig 5.12). This provides evidence that the overthrusting of the Morcles nappe is responsible for the deformation found within the cargneule. Despite the cargneule being the only Trias unit to show deformation all Trias formations must have undergone some slight metamorphism. The emplacement of the Morcles nappe would have buried the Trias under possibly a few kilometres of material; this would have produced a small increase in both pressure and temperature that can lead to metamorphism. However, the effects of very low-grade metamorphism on lithologies such as quartz arenites will be negligible. This burial would have produced a similar ‘sawtooth’ geothermal gradient as proposed by Oxburgh and Turcotte (1974), but not on such a great magnitude.

The two members found within the cargneule

The two members seen in the cargneule must reflect changes in depositional style or setting. An increase in water depth could explain the formation of the river valley white structureless Member (Fig. 4.21); this member was probably formed by the settling of very fine carbonate material in relatively deep water. The river mouth quartz limestone Member (Fig. 4.22) appears to be highly deformed, this is due to its close proximity to the decollement surface.

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Palaeoenvironmental analysis of the Trias

In order to obtain palaeoenvironmental models which are correct all of the formations and members must be considered together as a succession of correlable sediments. Determining palaeoenvironment for sandstone can be difficult since it can occur in a wide variety of different settings. The environment is unlikely to be shoreface since they are partially recognised on the basis of their fauna and the signpost sandstone Formation contains very little in terms of fauna. The sandstone comprises of coarse grained quartz arenites with well-rounded grains and shallow dipping laminations, these features are typical of beach facies. It is even possible to distinguish between lower and upper beach facies, the sandstone here displays characteristics of the lower beach facies since they are generally thick, moderatly to well-sorted and contain angular pebbles. Upper beach facies are generally thin with grain size and sorting increasing upwards. A reconstructed 3D palaeoenviornment model for the formation of the sandstone is shown below (Fig. 4.23):

Figure 4.23 - Reconstructed palaeoenvironment for the deposition of the sandstone

. The environments for the sandstone and the mudstone must be interconnected since the sandstone seen is present as thin layers within the mudstone and there is no visible unconformity between the two formations. There is a distinct lack of bioturbation in the mudstone; bioturbation would be expected if it had been deposited in a deep-water basin. This lack of faunal activity suggests that the mudstone has been deposited in a more terrestrial environment. A possible terrestrial environment that deposits mud is on the floodplain of a river; the mudstones in Emosson were probably deposited on the floodplain of a large, braided or sand-rich, river. This would explain the features observed within the mudstone. Since floodplain sequences are commonly associated with lake facies, large lakes that could have occupied the area can explain the presence of wave ripples. The presence of lakes can also explain the local variations in green and purple mudstone, the

Page 76 waterlogged conditions required for the formation of the green mudstone could have been provided by lakes on the floodplain. Areas dominated by green mudstone could, therefore, be the past positions of such lakes. The river was likely to be a particularly sandy river and sheets of sand were deposited during times of high precipitation and water level across the floodplain forming thin, continuous sand beds (Fig. 4.12). Starved sandstone ripples would have been the consequence of the same process, but the volume of sand deposited on the floodplain would have been insufficient to form coherent layers (Fig. 4.14). A 3D model showing the palaeoenvironmental depositional setting for the mudstone is shown below (Fig. 4.24):

Figure 4.24 - Reconstructed palaeoenvironment for the deposition of the mudstone

The presence of the angular polygonal hollows once occupied by halite and possibly gypsum in the cargneule point to an evapourite setting for the formation of the cargneule. The environment would have been very hot and arid. A 3D model showing the palaeoenvironmental depositional setting for the cargneule is shown below (Fig. 4.25):

Figure 4.25 - reconstructed palaeoenvironment for the deposition of the cargneule

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5. Morcles Nappe

The Morcles nappe is the youngest of all of the units within the area east of Lac d’Emosson (formed in the Jurassic). It is present directly above the Trias, and so is in contact with the cargneule’s uppermost member: the river mouth quartz limestone Member. Combe des Fonds carbonate nappe Formation

Stratigraphy and rock descriptions

The stratigraphic position of the Morcles nappe, which consists of just one formation: the Combe des Fonds carbonate nappe Formation, is shown in relation to the Trias and metamorphic basement in the section of GVS below (Fig. 5.1):

Combe des Fonds Triassic sediments carbonate nappe Formation

Tectonic thrust surface Unconformable top of the metamorphic basement (the Tete des Ottans augen gneiss Figure 5.1 - the Formation) GVS position of the Combe des Fonds carbonate nappe Formation

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Thickness – un-measurable, appears very thick but thickness has increased due to recumbent folding

Bedding – contains distinct alternating beds, these could easily be picked out and aided measuring folded structures

Grain size – fine

Sorting – very well sorted

Colour – grey-blue, sandstone present is creamy-yellow

Faunal content – none

Folding – contains a large number of folds, especially recumbent folds

Faulting – one fault was observed near fold five (Fig. 5.7)

The Combe des Fonds carbonate nappe Formation also contained medium- coarse grained, creamy-yellow layers of sandstone; some of these sandstone layers are deformed and are present as isolated lenses. The layers and lenses are sub-parallel to the foliation of the carbonate (Fig. 5.2).

Blue-grey carbonate

Creamy-yellow isolated sandstone lenses

Figure 5.2 - creamy-yellow sandstone lenses within fine- grained blue-grey carbonate rock (grid ref 5591310613)

Small (2-3mm) clasts of medium-coarse grained, creamy-yellow sandstone are also present in some areas (Fig. 5.3); some of these clasts are elongate parallel to the foliation. Diffuse sand-rich layers are also present within blue/grey carbonate rock (Fig. 5.3). The formation also contains many white-coloured calcite veins. An outcrop within the Combe des Fonds carbonate nappe Formation contains two intersecting foliations; this made the rock very crumbly. The main constituent of the Combe des Fonds carbonate nappe Formation is marl; this is a friable mixture of subequal amounts of micrite and clay minerals.

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Small sandstone clasts

Diffuse sandstone rich Lineations layers in carbonate nappe

Figure 5.3 - Photograph of nappe showing small sandstone clasts and lineations (grid ref 5591710539)

The Combe des Fonds limestone nappe Formation also contained lineations (Fig. 5.3); these lineations have the appearance of scratches or grooves on the surface of some of the outcrop surfaces. The average plunge and plunge direction of the measured lineations in the nappe is 20/298.

Structural observations within the Morcles nappe

As mentioned earlier, the Combe des Fonds carbonate nappe Formation is intensely folded and contains many recumbent folds; and there is also evidence for faulting. Seven of such folds were measured from distance (measuring the strike and dip of the limbs of the folds) to be used as a basis for comparison, correlation and for analysis. The seven folds are shown in the figures below (Figs. 5.4, 5.5, 5.7, 5.8 and 5.9):

Figure 5.4 - One of the large recumbent folds within the Morcles nappe, this is fold one (grid ref 5595510617)

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Fold two

Fold three

Figure 5.5 - Representative scale drawing of recumbent folds in the nappe using correct dips and thickness es. Shows folds two, three and four. The folds are drawn in relation to where they were in the exposed nappe section; Fig. 4.6 shows a photograph of where the folds were drawn

Fold four

Figure 5.6 - Photograph of Morcles nappe on the west side of Lac d'Emosson showing the face where the representative scale drawing was constructed (Fig. 5.5) (grid ref 5588310528)

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Fold five

Fault within the nappe

Figure 5.7 - Notebook sketch of fold five, this also shows a fault found within the Combe des Fonds carbonate nappe Formation (grid ref 5588910522)

Figure 5.8 - Photograph of fold six

Fold seven

Figure 5.9 - Notebook sketch showing fold seven and surrounding folding, numbers indicate dips measured from distance of the folded beds (grid ref 5587210504)

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Interpretations of the Morcles nappe

The Morcles nappe has been emplaced onto the Aiguilles Rouge massif and its thin overlying autochthonous Mesozoic sedimentary cover. The stratigraphy of the Morcles nappe consists of shallow-water successions with predominant massive limestones as well as sequences of marl and shale (Trümpy, 1980). Movement of the Morcles nappe provided the boundary conditions for simple shear deformation within the nappe itself. The major basal thrust of the nappe parallels bedding in marls and shales but cuts across competent limestone beds at 15º to 30º angles, in the direction of tectonic nappe geometry. The initiation of thrusts seems related to formation of ductile shear zones because thrusts are commonly observed to cut cleanly through the rock in a ductile manner (Ramsay et al, 1983). The role of basement shortening can also have a profound effect on the evolution of nappes. Basement shortening is linked with the formation of shear zones that narrow down the cover to form the bounding thrusts along which the nappes are tectonically emplaced (Ramsay et al, 1983).

Lineations found within the nappe

The lineations found in the nappe had an average plunge and plunge direction of 20/298, which roughly translates to the compass direction NNW. This is the transport direction of the Alps, so these lineations are probably to result of the movement of Alpine units.

Models of nappe emplacement

Three models are have been proposed to explain the emplacement of ductile folds or thrust nappes: gravity slide, a horizontal compression and gravitational collapse. The strain found in the Morcles nappe is reasonably consistent with the gravity slide model (Fig. 5.10). However, the inhomogeneity of strain and complex history of deformation makes comparisons with such simple models imprecise (Twiss and Moores, 2000).

Figure 5.10 - Models of nappe emplacement (a) portion of the nappe before deformation (b) the gravity slide model, the likely model for the Morcles nappe (modified from Twiss and Moores, 2000)

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Fold development in the nappe

Ramsay (1983) stated that some folds, initially of low amplitude:wavelength ratios develop from movement of the nappe over the underlying ramp-flat surface. However, most folds form because of the effects of simple shear within the nappes or because of the modification of the geometry of ramp folds by simple shear. During simple shear, strata parallel to the boundaries of the nappe undergo translation without shortening. Strata inclined across the nappe boundaries undergo different amounts of shortening, depending on their inclination to the nappe boundaries, which act as shear-zone boundaries. Buckle folds form in competent layers as a result of this shortening (Fig. 5.11) (Ramsay et al, 1983).

Figure 5.11 - Models for nappe emplacement, the second diagram shows how the geometry is modified by heterogeneous strains within the nappe and the formation of buckle folds (after Ramsay et al, 1983)

Interpretations of measured fold hinges

The measured fold limbs of recumbent nappe folds were plotted on stereonets to find their hinge lines (see appendix), the calculated hinge lines for the seven folds can be plotted onto a stereonet (Fig. 5.12). The resultant stereonet clearly shows that the hinge lines for the folds are not the same, which suggests a more complex evolution for the folds. The hinge lines are shown to line up on both a small circle and a great circle.

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Figure 5.12 - Stereonet to show the relationship between the calculated hinge lines of the folds measured in the nappe

The variation in hinge lines is likely to be associated with shearing, indicating that the hinge lines of the folds were formed in close proximity to a shear plane. This has caused the ‘shear out’ of the existing hinge lines into a tongue shape; these types of folds are called sheath folds that are often non-cylindrical or sheath-like in appearance. A simplistic diagram showing the possible formation of the sheath folds is shown in Fig. 5.13.

Figure 5.13 - Simplistic diagrams showing the 'shear out' of a hinge line to form a sheath fold

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Hudleston (1977) has clearly shown that a necessary condition for the formation of sheath or passive folds it that the layering must be oblique to the streamlines. Cobbold and Quinquis (1979) stated that there are three different models presented for the development of sheath folds (Fig. 5.14). The first two invoke steady streamlines, whereas the third invokes unsteadiness accompanying boudinage. The resulting sheath folds are strongly asymmetric, with curved fold hinges. Geological examples of sheath folds can occur in many natural shear zones and are formed in an essentially passive manner (Cobbold and Quinquis, 1979). They suggested that at the base of a nappe folds develop by passive amplification of initial deflections in an otherwise planar and passive layering which is grossly parallel to the shearing plane. The three models proposed by Cobbold and Quinquis (1979) are as follows:

Figure 5.14 - Models for the formation of sheath folds, the likely model in the case of a fold nappe is model one

Metamorphic grade of the Morcles nappe

The Morcles nappe, similar to the metamorphic basement, has also been metamorphosed as part of its evolution. The nappe shows signs of metamorphism and deformation, such as deformed sand lenses (Fig. 5.2) and intersecting foliations. Kirschner, Sharp and Masson (1995) investigated such metamorphism; they found that mineral paragenesis, illite ‘crystallinity’ and fluid inclusion chemistry record an apparent peak metamorphic temperature gradient that increased across the Morcles nappe from anchizonal in the foreland to epizonal in the hinterland root zone. They analysed twenty-seven quartz-calcite veins and found that peak metamorphic temperatures during formation and deformation ranged from ~260-290°C in the shallower, foreland localities and from ~330-350°C in the deeper, more hinterland localities. Therefore, the Morcles nappe, similar to the basement, also experienced subgreenschist metamorphism.

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6. Superficial deposits

Drift is present where there is no solid geology at the surface. Solifluction or head deposits (created by soil moving downhill via gravity) are present over most of the rocks to a greater or lesser extent in the Emosson area. The basement rocks contain a thin cover of solifluction deposits with many protruding outcrops (Fig. 6.1); this, however, has not been classed as drift. Most of the Triassic sediments are present underneath solifluction deposits and possibly fluvial deposits since the area contains many mountain streams; good outcrops of the Trias are present in eroded stream beds where there is little soil. The area also contains a lot of loose angular scree that has been weathered from the surrounding rocks, probably by the alternating freezing and thawing of contained water. Scree is mainly present around the edges of steep metamorphic or nappe areas, such as at grid ref 5602510391. The area also contains possible evidence of glacial erosion that can be seen on the map surrounding the grid ref 5601810541. Salanfe area two contains much evidence for glacial erosion in the form of glacial striations and moraine.

Figure 6.1 - Photograph of the metamorphic basement on the east side of Lac d’Emosson area showing extent of superficial deposits

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7. Geological history

As with every orogenic area, the geological history of the Emosson area is extremely complex. This section outlines the sedimentological, metamorphic and structural history of the area based on the evidence presented in the previous sections. The geological history of the area is presented in a list below, from the oldest event to youngest:

• Pre-Variscan sediments are deposited, probably as shales and muds in a deep- water basin.

• These sediments are buried to about 30km depth (von Raumer and Schwander, 1985) calculated an early pressure peak of 10kbar) and experience subgreenschist facies metamorphism; the majority of the small-scale basement faults are formed before the metamorphic peak of amphibolite facies is reached. This is due to the fact that the minerals present as mineral fiber lineations on the fault surface are not of high-greenschist or amphibolite facies.

• The main foliation forms from previous bedding in the now metamorphic rocks.

• Ductile deformation occurs causing folding on the metre and km-scale, which folds the main foliation; more faulting could have also occurred as a consequence of this folding.

• The metasediments are brought to the surface; a likely mechanism for this is exhumation.

• The metasediments start to cool.

• The Chapelle d’Emosson white granite Formation is intruded in late-Variscan to the south of the metamorphic basement on the east side of Lac d’Emosson.

• A period of non-deposition and erosion followed, leading to the formation of the unconformity and the weathering of the top of the Tete des Ottans augen gneiss Formation (feldspar augens are weathered red/pink).

• A cohesive, terrestrial debris or mud flow containing very large angular quartz crystals spreads out over a laterally extensive area.

• Sand is deposited on top of the thin debris flow layer in beach setting in the Triassic; this beach is frequented by dinosaurs.

• Sea level falls and mud is deposited on top of the sandstone on the floodplain of a sandy, braided river. This change is not rapid since the interbedded member signals the slow transition between the two environments.

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• The muds and sands lithify by compaction due to the weight of overlying sediment.

• The Cargneule is deposited on top of the mudstone in an arid, sabkha environment; this occurred after lithification of the mud since pieces of mudstone from the area have been incorporated into the cargneule.

• Onset of the Alpine orogeny: the Morcles nappe thrusts over the top of the metamorphic basement and overlying Trias in the Oligocene. This has caused deformation in the underlying cargneule.

• Erosion of the formations in the area occurs, which has resulted in the topography that is seen in the present day.

The result of this complicated evolution is shown in the final GVS of the whole area and the overall cross section which transects through all the formations found in the Emosson area (the cross section goes from grid ref 5614410197 to 5595110419 and from 5598710444 to 5593410518) (Figs. 7.1 and 7.2):

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Figure 7.1 – Final cross section for the entire Emosson area showing all the formations

Figure 7.2 – final Page 90 GVS for the Combe des Emosson area Fonds carbonate nappe Formation

Triassic formations (shown on separate GVS’s in Figs. 4.1 and 4.15)

Tete des Ottans augen gneiss Formation

Combe du Col mica schist Formation

Barrage de Barberine garnet schist Formation

Barrage de Barberine augen gneiss Formation

North of the big tunnel layered metasediment Formation

North of the big tunnel garnet gneiss Formation South of the small tunnel mica schist Formation North of the small tunnel layered metasediment Formation Small tunnel feldspar gneiss Formation

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Acknowledgements

Thanks are given to D. Prior and D. Faulkner for helpful discussions and advice and to J. Wheeler for kindly providing regional geology maps for the western Alps. Thanks must also go to A. Lennie, for providing data from the west side of Lac d’Emosson and for general help and company while we were camped in the area. Further thanks are given to R. King and G. Potts for spending time to help the project and the Department of Earth and Ocean Sciences, Liverpool for financial support. A special mention must go to Frank Lugon-Moulin, the owner of the Restaurant de Gueulaz, for his generosity and hospitality.

References

Amieux, P., Jeanbourquin, P. 1989. Cathodouminescence et origine diagenetique tardive des cargneules du massif des Aiguilles Rouges (Valais, Suisse). Bulletin de la Societe geologique de France, 1, 123-132. Brandlein, P., Nollau, G., Sharp, Z., von Raumer, J. 1994. Petrography and geochemistry of the Vallorcine granite (Aiguilles Rouges Massif, Western Alps). Schweizerische mineralogische und petrographische mitteilungen, 74, 227-243. Cobbold, P. R., Quinquis, H. 1979. Development of sheath folds in shear regimes. Journal of structural geology, 2, 119-126. Dobmeier, C. 1998. Variscan P-T deformation paths from the southwestern Aiguilles Rouge massif (External massif, western Alps) and their implication for its tectonic history. Geologische Rundschau, 87, 107-123. Dobmeier, C., von Raumer, J. F. 1995. Significance of latest-Variscan and Alpine deformation for the evolution of Montagne de Pormenaz (Southwestern Aiguilles Rouge Massif, Western Alps). Eclogae Geologicae Helvetiae. 88, 267- 279. Grandjacquet, A. and Haccard, D. 1975. Analyse des sediment polygeniques neogenes a facies cargneules associes a des gypses dans les Alpes du Sud. Extension de ces facies au pourtour de la Mediterranee occidentale. Bulletin of the Geological Society of France, 17, 242-259. Huddleston, P. J. 1977. Similar folds, recumbent folds, and gravity tectonics in ice and rocks. J. Geol. 85, 113-122. Kirschner, D. L., Sharp, Z. D., Masson, H. 1995. Oxygen isotope thermometry of quartz-calcite veins: Unravelling the thermal-tectonic history of the subgreenschist facies Morcles nappe (Swiss Alps). GSA Bulletin, 10, 1145-1156. Liesa, C., and Lisle, R. 2002. Reliability of methods to separate stress tensors from heterogeneous fault-slip data. Journal of structural geology, 26, 559-572. Marshall, D., Kirschner, D., Bussy, F. 1997. A Variscan pressure-temperature-time path for the N-E Mont Blanc massif. Contributions to mineralogy and petrography, 126, 416-428. Oxburgh, E. R., and Turcotte, D. L. 1974. Thermal gradients and regional metamorphism in overthrust terrains with special reference to the Eastern Alps. Pilloud, C. 1991. Structures de deformation alpines dans le synclinal de Permo- Carbonifere de Salvan-Dorenaz (massif des Aiguilles Rouge, Valais). Memoires de Geologie (Lausanne), 9, 101.

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Appendix

Stereonets used to calculate the hinge lines for the seven folds measured in the Morcles nappe are as follows:

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